This invention relates generally to ultrafiltration technology and, more particularly, to a spiral wound filtration module for use in cross-flow filtration and to a method of constructing same.
The term "ultrafiltration" as used in the present application is intended to encompass microfiltration, nanofiltration, ultrafiltration and reverse osmosis and gas separation. A typical ultrafiltration device comprises a plurality of spiral wound filtration modules through which a fluid to be filtered passes. Such a module is made by winding one or more membrane leaves and permeate envelopes around a permeate tube. The membrane leaves are separated by feed spacer screens which are of a relatively large mesh size to accommodate fluid flow. The permeate passes through the membrane surface of the membrane leaves and is directed to the permeate tube by a permeate carrier sheet. Some type of external restraining means such as a hard shell, straps or a bypass screen, or a combination thereof may be used to hold the spirally wound leaves in tight formation around the tube. The spiral module is then loaded into a housing or pressure vessel which is operated at a slight pressure drop across the module as the fluid being filtered flows through. Concentrate is removed from one end of the module and permeate is removed from the permeate tube
Many applications of ultrafiltration technology involve food processing where sanitary conditions must be maintained at all times. This necessitates periodic cleaning with relatively harsh chemicals such as (by way of example only) chlorine containing compounds, other oxidizing agents, acids, alkalies and surfactants. These chemicals tend to degrade the membrane material, particularly in areas that are subject to stress. A typical procedure for constructing spiral filtration modules includes folding a membrane sheet in the area that is to be adjacent to the permeate tube. This fold area creates mechanical stresses in the membrane sheet both at the crease and at the point of the contact with the adjacent permeate carrier sheet. Other stress areas in a spiral wound membrane include the location of overlap between two membrane leaves and the overlap of the membrane with any underlying stitching or mechanical fastening devices.
It is typical to employ some type of reinforcing in the fold area so as to reduce the mechanical stresses and prolong the life of the membrane. Two primary techniques are well known to those skilled in the art. The first is the utilization of reinforcing tape which is applied at the crease and extends outwardly from the crease a short distance over what is typically referred to as the fold area of the membrane. The second method of membrane reinforcement is to apply an adhesive in generally the same area as that to which the tape is applied and for the same effect. An example of this second method is contained in the Bray, et al. U.S. Pat. No. 4,842,736. This patent further discloses a modification of the second method wherein, in place of a flowable adhesive, a soft melt thermoplastic material is employed from the backing side of the membrane to fill the interstices of the backing material and penetrate the thickness of the backing all the way to the actual membrane material.
The difficulties with these prior art techniques for strengthening the fold area of a spiral membrane are that the tape tends to eventually lose its adhesion and peel away, and glue is applied at a thickness such that, while the membrane is strengthened in the fold area, the glue has a tendency to create new stress points especially along its terminal edge. Both prior art techniques increase the thickness of the membrane leaf at the line of transition between reinforced and unreinforced membrane which is also a factor in introducing new stress points. Also, when either tape or glue is applied to the membrane surface (as opposed to the membrane backing), failure of either material may expose a "dead area" between the membrane surface and the failed glue or tape where the fluid being filtered may collect causing sanitation and eventual leakage problems. If the reinforcing is applied to the backing of the membrane, as contemplated in the referenced patent to Bray et al., there is no protection against surface cracks in the membrane itself at the crease or fold creating small crevices where fluid can collect and under some conditions create sanitation problems.